1. Field of the Invention
The present invention relates to a microcomputer and a burn-in test method thereof and more particularly a microcomputer including a burn-in test circuit and a burn-in test method thereof.
2. Description of the Related Art
There has been known a microcomputer which is provided with a testing function by which a test mode for testing its operation can be set from the outside, in addition to the user mode for the use by the user. As the operation test of the microcomputer, a burn-in test by which the initial defects of the microcomputer are checked by operating it under the conditions of application of thermal and electrical stresses is generally employed.
An example of the conventional microcomputer for which the test mode can be set from the outside is shown in FIG. 5.
A microcomputer 5 is composed of a central processing unit (CPU) 51, a ROM 52, a mode decoder 54, an address bus 56, a data bus 57, and a NOR circuit 61. The ROM 52 is composed of a user ROM 52a, a test ROM 52b, an address decoder 53, AND circuits 63 and 64, and a NOT circuit 62. The user ROM 52a and the test ROM 52b respectively have address lines corresponding in numbers to their addresses, and access to them can be gained when the address lines are activated.
The test mode for the microcomputer 5 can be set at either one of ROM dump mode (for a ROM dump mode signal S1 of "1"), burn-in test mode (for a burn-in test mode signal S2 of "1"), or test ROM execution mode (for a test ROM execution mode signal S3 of "1") according to the mode set signals input to mode set terminals 54a to 54c.
In the burn-in test, since the voltages applied to respective terminals of the microcomputer 5 are determined according to the wiring connection of the burn-in test device on which the microcomputer 5 is mounted, the mode set signals input to the mode set terminals 54a to 54c of the microcomputer 5 are determined accordingly. It is true that the turn-in test mode signal S2 and the test ROM execution mode signal S3 are effectively the same, so that the burn-in test can also be carried out by setting the test ROM execution mode.
In the burn-in test mode, the burn-in test mode signal S2 alone is set exclusively at "1" (high level) by the mode set signals determined in accordance with the wiring connection of the burn-in test device. In this case, an address signal TAD is supplied to the address lines of the test ROM 52b via the NOR circuit 61, the NOT circuit 62, and the AND circuit 63, and the CPU 51 gains access to the test ROM 52b according to the address decoded by the address decoder 53. In this way, the test program in the test ROM 52b is executed.
Here, the test program is for operating various parts of the microcomputer 5. Namely, while letting the CPU 51 execute the test program in the test ROM 52b, it lets various sections of the microcomputer 5 operate under the conditions of application of thermal and electrical stresses to perform the burn-in test. After the completion of the burn-in test, the program in the test ROM 52b is executed in the test ROM execution mode (where no thermal stress is applied to the microcomputer 5), and initial defects in the microcomputer 5 are detected by verifying the data output via an external input/output terminal 56a of the address bus 56 and an external input/output terminal 57a of the data bus 57.
However, in the conventional microcomputer 5 described above, in the burn-in test mode, access to the user ROM 52a is not gained since an address signal UAD is not supplied to the address lines of the user ROM 52a. That is, in the burn-in test mode, the user ROM 52a is not subjected to an electrical stress. Accordingly, even if a burn-in test is carried out by mounting the microcomputer 5 on the burn-in test device, and storage contents of the user ROM 52a is dumped under the condition where the normal thermal stress is not applied, it is not possible to fully detect the initial defects of the user ROM 52a.
In the meantime, it is conceivable to make the mode set signals input to the mode set terminals 54a to 54c to serve as signals for the ROM dump mode in order to gain access to the user ROM 52a, and carry out the burn-in test of the microcomputer 5 in this state.
However, in this case there is required a burn-in test device for giving a mode set signal in the ROM dump mode, in addition to the burn-in test device for giving a mode set signal in the burn-in test mode. Moreover, it becomes necessary to demount the microcomputer 5 from the burn-in test device after the completion of the execution of the test program in the burn-in test mode, and mount it on a separate burn-in test device, which requires much labor.
As a microcomputer having the conventional burn-in test function one may mention the microcomputer disclosed in Publication of Unexamined Patent Applications No. Hei 4-238543. However, this invention discloses only the execution of a test program at the time of burn-in test, similar to the above, and no method of access to the user program storage area is disclosed.